1. Field of the Invention
The present invention relates to dielectric resonators, dielectric filters, and dielectric duplexers which have plated through holes arranged inside dielectric blocks and outer conductors formed on the outer surfaces of the dielectric blocks. In addition, the invention relates to communication apparatuses incorporating the same.
2. Description of the Related Art
In a conventional dielectric resonator, a plurality of plated through holes (holes having inner conductors formed on the inner surfaces thereof) are formed inside a substantially rectangular parallelepiped dielectric block having an outer conductor arranged on the outer surfaces of the dielectric bock. One end of each of the plated through holes is open-circuited and the remaining end of each through hole is short-circuited.
The inner diameter of the short-circuited end of each plated through hole differs from the inner diameter of the open-circuited end thereof to form a stepped structure. With this structure, the axial length of each plated through hole is shortened. The stepped structure will be described with reference to FIGS. 14A and 14B. FIG. 14A shows an external perspective view of a dielectric resonator and FIG. 14B shows a sectional view of the resonator.
In FIG. 14A, the reference numeral 1 denotes a substantially rectangular parallelepiped dielectric block. Plated through holes 2a and 2b extend from the left-front surface of the dielectric block to the right-rear surface thereof. The left-front surface in the figure is set as an open face, and, except for this surface, an outer conductor 4 is formed on substantially all of the remaining five surfaces of the dielectric block 1. Input/output electrodes 5a and 5b are arranged on outer surfaces of the dielectric block 1 and are isolated from the outer conductor 4. When the resonator is surface-mounted, the top surface shown in FIG. 14A is mounted on a circuit board (i.e., the top surface faces the mounting surface of the circuit board) and the input/output electrodes 5a and 5b are electrically coupled to electrodes arranged on the circuit board.
As best shown in FIG. 14B, the plated through holes 2a and 2b have stepped structures formed by forming steps located inside the dielectric block 1. With this stepped structure, as compared with a dielectric block including plated through holes 2a and 2b having substantially fixed inner diameters, the length of the holes can be reduced for a given wavelength. In this case, since the inner diameter of the open-circuited end (the left end in FIG. 14B) of the plated through hole is larger than the inner diameter of the short-circuited end (the right end as viewed in FIG. 14B) of the plated through holes, the thickness D between the outer surfaces and the wide diameter section of the plated through holes is small.
Next, other conventional dielectric resonators will be discussed with reference to FIGS. 15A and 15B. FIGS. 15A and 15B illustrate perspective views of two different dielectric resonators.
In both dielectric resonators, a plated through hole 2 extends from one surface of a dielectric block 1 to the opposing surface thereof. An outer conductor 4 is arranged on substantially the entire outer surface of the dielectric block 1. The inner conductor at the upper right end of the plated through hole 2 (the end not seen in the figures) is directly coupled to the outer conductor 4 forming a short-circuited end. The inner conductor at the other end of the plated through hole 2 is capacitively coupled to the outer conductor 4 forming an open-circuited end. In the embodiment of FIG. 15A, an outer coupling electrode 16 is directly coupled to the inner conductor on the plated through hole 2 but is isolated from the outer conductor 4. In this situation, the outer coupling electrode 5 extends from the open-circuited end of the plated through hole to a mounted surface on which the resonator is mounted.
In the dielectric resonator shown in FIG. 15B, on the open-circuit-end face (the left-front surface shown in the figure) of the plated through hole 2, an open-circuited end electrode 61 is directly coupled to the inner conductor of the plated through hole 2 and is capacitively coupled to both the outer conductor 4 and the outer coupling electrode 5. The outer coupling electrode 5 is isolated from the outer conductor 4 and is electrically coupled to a signal line on the circuit board on which the dielectric resonator is mounted.
A significant problem with these conventional dielectric resonators is that electromagnetic waves leak at the open-circuit end of the dielectric block. The leakage of the electromagnetic waves reduces the amount of ground current, thereby deteriorating filter attenuation characteristics. In order to prevent such deterioration, a cover for the open-circuited end is required.
In addition, in the structure shown in FIG. 15B, since coupling between the resonator defined by the plated through hole and the outer coupling electrode is performed only near the open-circuited end, the maximum coupling capacity is small, thereby narrowing the range of the obtainable coupling capacity.
In terms of the outer configuration, by using the stepped structure, the height of the dielectric block 1 can be reduced. However, when the height of the block 1 is 1.5 mm or less, the thickness of the dielectric block on the open-circuited-end side becomes smaller than the thickness of the possible formation limit. As a result, it is difficult to form the dielectric block 1.